1. Field of the Invention
The present invention relates to a bidirectional switch capable of bidirectionally turning on/off a current, and particularly, to a technique of suppressing a bidirectional overvoltage with an additional simple circuit.
2. Description of the Related Art
A power conversion apparatus employing a power semiconductor switch to convert AC or DC power of a certain level into AC or DC power of another level is used for uninterrupted power sources, motor inverters, communication DC power sources, and the like.
Generally, the power semiconductor switch is vulnerable to an overvoltage and will break if receiving, even momentarily, an overvoltage exceeding a withstand voltage of the power semiconductor switch. To prevent the overvoltage, there is a technique of connecting a series circuit of a constant voltage diode and a resistor between the drain and gate of the power semiconductor switch.
FIG. 1 is a circuit diagram illustrating a semiconductor switch disclosed in Japanese Unexamined Patent Application Publication No. 2004-079892. This semiconductor switch is a power semiconductor switch provided with an overvoltage protection circuit. In FIG. 1, the semiconductor switch FET Q1 has a drain and source that are connected through terminals 11 and 12 to a main circuit (not illustrated). The drain of the FET Q1 is connected to an anode of a diode 17. A cathode of the diode 17 is connected through a resistor 16 to a cathode of a constant voltage diode (Zener diode) 15. A breakdown voltage of the constant voltage diode 15 is lower than a withstand voltage of the FET Q1.
An anode of the constant voltage diode 15 is connected to a gate of the FET Q1 and a first end of a resistor 14. A gate signal unit 13 is connected to a second end of the resistor 14 and the source of the FET Q1. A series circuit including the diode 17, resistor 16, and constant voltage diode 15 is connected between the drain and gate of the FET Q1, to form the overvoltage protection circuit.
The gate signal unit 13 turns on the FET Q1 in response to an ON signal from a control circuit (not illustrated), so that an anode voltage is supplied to the terminal 12. In response to an OFF signal from the control circuit, the gate signal unit 13 turns off the FET Q1, so that a cathode voltage is supplied to the terminal 12.
Operation of the related art illustrated in FIG. 1 when an overvoltage occurs between the drain and source of the FET Q1 will be explained.
When the gate signal unit 13 applies a cathode voltage through the resistor 14 to the gate of the FET Q1, the FET Q1 turns off and a drain voltage of the FET Q1 increases. If a drain-gate voltage of the FET Q1 exceeds the breakdown voltage of the constant voltage diode 15, a current is supplied from the drain of the FET Q1 to the gate signal unit 13 through the diode 17, resistor 16, and constant voltage diode 15, to increase a gate voltage of the FET Q1.
If the increased gate voltage of the FET Q1 causes a gate-source voltage thereof to exceed a gate threshold voltage of the FET Q1, the FET Q1 turns on to pass a drain-source current and decrease the drain voltage, thereby preventing a drain-source overvoltage of the FET Q1.
Japanese Patent No. 4123274 discloses an AC switch (bidirectional switch) to turn on/off an AC signal. This AC switch is a compound semiconductor such as silicon carbide (SiC) or gallium nitride (GaN). As illustrated in FIG. 2, this related art connects an FET Q2 serving as a normally-ON switch between terminals 11 and 12. The FET Q2 has main electrodes 21 and 22 and a gate G. The normally-ON FET Q2 has symmetrical drain and source, and therefore, one of the main electrodes 21 and 22 connected to one of the terminals 11 and 12 that has a higher voltage than the other terminal becomes a drain and the other main electrode becomes a source.
A first end of a gate signal unit 13 is connected to the gate of the FET Q2. The main electrode 21 of the FET Q2 is connected to a cathode of a diode D1 and the main electrode 22 of the FET Q2 is connected to a cathode of a diode D2. Anodes of the diodes D1 and D2 are connected to a second end of the gate signal unit 13.
When an AC signal is inputted between the terminals 11 and 12 and when a voltage at the terminal 11 is higher than that at the terminal 12, the main electrode 21 of the FET Q2 becomes a drain and the main electrode 22 a source. If a gate signal is supplied to make a gate voltage of the FET Q2 equal to or higher than the voltage at the main electrode 22, i.e., the source of the FET Q2, the FET Q2 turns on to turn on the diode D2. At this time, the diode D1 is reversely biased and turns off.
If the voltage at the terminal 12 is higher than that at the terminal 11, the main electrode 21 of the FET Q2 becomes a source and the main electrode 22 a drain. If a gate signal is supplied to make the gate voltage of the FET Q2 equal to or higher than the voltage at the main electrode 21, i.e., the source of the FET Q2, the FET Q2 turns on to turn on the diode D1. At this time, the diode D2 is reversely biased and turns off.
Even if the voltage at the terminal 11 is higher than that at the terminal 12, or even if the voltage at the terminal 12 is higher than that at the terminal 11, the FET Q2 turns off if a gate signal is supplied to make the gate voltage of the FET Q2 lower than the voltage at the main electrode serving as a source.
Another related art concerning a semiconductor switch is disclosed in Japanese Unexamined Patent Application Publication No. H10-261945.